It’s not working because the entire for loop (from the for to the final closing parenthesis, including the commands between those) is being evaluated when it’s encountered, before it begins executing.
In other words, %count% is replaced with its value 1 before running the loop.
What you need is something like:
setlocal enableextensions enabledelayedexpansion
set /a count = 1
for /f "tokens=*" %%a in (config.properties) do (
set /a count += 1
echo !count!
)
endlocal
Delayed expansion using ! instead of % will give you the expected behaviour. See also here.
Also keep in mind that setlocal/endlocal actually limit scope of things changed inside so that they don’t leak out. If you want to use count after the endlocal, you have to use a “trick” made possible by the very problem you’re having:
endlocal && set count=%count%
Let’s say count has become 7 within the inner scope. Because the entire command is interpreted before execution, it effectively becomes:
endlocal && set count=7
Then, when it’s executed, the inner scope is closed off, returning count to it’s original value. But, since the setting of count to seven happens in the outer scope, it’s effectively leaking the information you need.
You can string together multiple sub-commands to leak as much information as you need:
endlocal && set count=%count% && set something_else=%something_else%